FIELD OF THE INVENTION
The invention relates to an electricity meter having at least one differentiating input converter, an analog/digital converter downstream of the at least one input converter, one analog filter connected between each respective input converter and the analog/digital converter, a device downstream of the analog/digital converter for digital signal processing, and a digital filter for digital signal processing.
Such an electricity meter is known from U.S. Pat. No. 4,992,725.
In an input circuit of an electricity meter which is known from German Published, Non-Prosecuted Patent Application DE 87 27 053 A1, a differentiating input converter is followed by an integrator with direct-voltage negative feedback. The integrator serves to compensate for the differentiating effect of the input converter. In that example, the integrator is constructed as an operational amplifier with RC wiring in the negative feedback branch. The disadvantage of such circuits is their in long-term drift and the temperature drift in the integration capacitor disposed in the negative feedback branch. In addition, magnetic and electric interference represent a problem for the integrator.
In constructing the meter with digital signal processing, the problem also arises that the components used there are very vulnerable to interference. That applies particularly to voltage peaks, of the kind that occur in differentiating converters, for instance.
U.S. Pat. No. 5,315,236 discloses an electricity meter intended for connection to a differentiating input converter. The input converter is followed by a digital/analog converter and a device for digital signal processing. Connected between the input converter and the digital/analog converter is an analog filter, which is constructed as an active low-pass filter with an amplifier. The device for digital signal processing, constructed as a CPU, is used to calculate current consumption from input data.
The use of sigma-delta converters and multiplexers in three-phase electricity meters is known in principle from European Patent Application 0 634 662 A1.